This invention relates to a printed wiring board and manufacturing method thereof, and more particularly to a printed wiring board for use in an electronic wrist watch, which requires an extremely high accuracy of circuit pattern.
Recently, the electronic wrist watch has become popular. In such wrist watch, a miniaturized printed wiring board is used combined with a semiconductor device such as integrated circuit (IC) and large scale integrated circuit (LSI). The semiconductor device must be electrically connected to an array of contact regions on the wiring board. Therefore, it is required to arrange the contact regions accurately and at a high density around the semiconductor device. The electrical connections between the contact regions and corresponding contact pads of the device are performed by a wire bonding method using fine metal line such as gold and aluminum. To meet this method, the contact regions, which are generally made of copper, are electroplated with desired metal as an overplated coating.
The invention contemplates effective solution of various problems involved in such overplating technique.
In the overplating technique, such novel metals as gold, gold on nickel, are used. However, since such novel metals are expensive it is necessary to minimize the quantity of the overplated metal.
A typical prior art method of overplating will firstly be described with reference to FIGS. 1a through 1e.
According to the prior art method, as shown in FIG. 1a, a conductive metal pattern 12 is formed on a desired portion of an insulating substrate 11 by an ordinary method of making printed wiring board. The conductive pattern 12 can be divided into two areas. One is a circuit area and the other is an electroplating current supply area. The circuit area includes contact regions to be overplated which are indicated as contact tabs A1, A2, A3, A4 and C, and lands B1, B2 and B3. The circuit area further includes circuit leads designated as D2 and D3. On the other hand, the electroplating current supply area consists of electroplating leads designated as E, D1, D4, D5, D6 and D7.
In the next step, a conventional plating resistant organic insulating film, is selectively coated on the substrate except for the contact regions A, B and C, and electroplating lead E as shown by a shaded area in FIG. 1b.
Then, as shown in FIG. 1c, an overplated plating 14, which consists of gold, solder or double layers of gold on nickel is formed on the exposed contact regions A, B and C and an exposed portion of the electroplating lead E. The plating current is supplied through the electroplating lead E formed on a margin area to be removed in the product.
Thereafter, a mechanical cutting operation is performed on the wiring board to isolate the tabs A1 to A4 and C from each other, in addition to the isolation of the lands B1 and B2 from the lead E. Of the resist film 13 is an adhesive tape coated with an adhesive agent, it is required to be removed before the mechanical cutting operaton. In the above drawings, it is assumed that an organic insulating film is used to avoid the peeling process of the film 13. In FIG. 1d, drilling or press punching is used to form six openings 161 to 166 at the connecting portions between the leads D1 and D4 to D7, and between the lead E and the lands B1 and B2.
In FIG. 1e, on the other hand, a milling cutter process is used to form an elongated slot 17 along the lead D1, while removing the lead E along a dotted line designated as F.
However, the cutting operation described above accompanies the following problems.
When a drill is used, the drill is rotated in one direction and advanced into the substrate 11 in a direction perpendicular to the surface of the metal pattern 12. Thus, as shown in FIG. 2c, "burrs" or "beards" 12a and 12b are formed on the walls of the openings in a rotating direction of the drill due to the viscosity of metal. For this reason, it becomes harder to electrically isolate the tabs A1 to A4 from each other unless the diameter of each opening is enlarged.
Furthermore, since the substrate 11 usually consists of glass-epoxy laminate, irregularities 11a are formed on the walls of the drilled opening. In a microscopic view, the laminated glass clothes and the synthetic resin films are separated due to mechanical shock applied at the time of drilling, thus imparing the characteristics of the insulating substrate. In the same manner, the milling cutter forms irregularities 11a' and burrs on beards 12a' on the wall surface as shown in FIG. 2b. Therefore, it is inevitable to cause the aforementioned shortcomings.
When a press punching is used instead of the drill, beards and burrs 12a" and 12b" are also formed downward on the walls of punched opening as shown in FIG. 2c and some of these irregularities are deeply extended.
If the insulating substrate 11 is thin, the metal patterns 12 on the upper and lower sides thereof are short-circuited through the punched opening or the effective diameter of the opening becomes smaller thus degrading the electrical characteristics of the insulating substrate.
When the printed wiring boards made by these prior art methods are used in a high humidity environment, there is a tendency of forming dew on the fractured surfaces because of their irregularity. Accordingly, electric insulaton strength between the metal patterns 12 is reduced unless the distances between these patterns are widely arranged.
Moreover, since the foregoing mechanical processes have a limit in its working accuracy, a time circuit pattern is hardly obtained.